1. Field of the Invention
The present invention relates to a new method for obtaining conductive polymers that can be used to increase their conductivity and make it thermally stable.
The obtaining of high thermal stability then makes it possible to use such polymers for industrial purposes since their use generally necessitates steps in which a heating operation most usually leads to a decrease in their conduction.
2. Description of the Prior Art
At present, conductive polymers are obtained by several methods of synthesis.
The electrochemical method has been the one most studied since it can be used to synthesize polymers that perform well, but this type of synthesis generally results in the obtaining of a film and not a powder. The level of efficiency and the quantities inherent in this method of synthesis means that it is difficult to exploit it for industrial purposes. More precisely, the electrochemical method consists of an electropolymerization during which the polymer develops on an electrode and is made conductive by the insertion of ion species that stabilize the conduction. This mode of synthesis enables a great variety of polymerization both at the monomer level (pyrrole, thiophene, aniline, indole etc) and at the level of stabilizing species.
During the process of growth of the polymer on the anode, the oxidizing of the polymer leads to the creation of an electron defect (p type conduction). In the case of pyrrole, for example, this error is of the order of one hole for three heterocyclic structures. ##STR1##
The electrolyte A present gets dissociated and stabilizes the p type conduction, in forming a complex on the chain. The heat stability of the conduction of the conductive polymer then results mainly from the stability of the fixing of the anion.
The best results have been obtained on crude conductive polymer by means of paratoluene sulfonate, phenylsulfonate, alkylfluorosulfonate anions as compared with anions of low steric hindrance such as BF.sub.4.sup.-, PF.sub.6.sup.-, ClO.sub.4.sup.-. Furthermore, it would appear that the group RSO.sub.3 has an electronegative potential greater than that of the anions BF.sub.4.sup.-, PF.sub.6.sup.-, ClO.sub.4.sup.- or RC00-: this furthers its quality of gripping in the polymer matrix.
At the same as the studies on the electrochemical method, work is being done on oxidative polymerization. This type of purely chemical synthesis brings a redox pair into play and enables the making of a conductive powder with submicronic sizes (1 .mu.m to 0.1 .mu.m), and achieves this result with excellent efficiency.
The reaction process generally brings together an oxidizing agent of the FeCl.sub.3, Fe(NO.sub.3).sub.3 or CuCl.sub.2 type, which will get reduced in the presence of a monomer.
Having been oxidized, the monomer gets polymerized in short chains (20 to 50 monomer units). The anion provided by the oxidizing agent behaves similarly to the anion of the electrolysis. For example, in the pair formed by pyrrole and ferric chloride, the oxidation-reduction reaction is as follows: ##STR2##
The chain thus synthesized therefore has anions of low hindrance and low electronegativity. The semiconductor powders are not very stable thermally and in relation to the oxidation. For example, polypyrrole powders cannot withstand being heated to more than 80.degree. C. for some hours. Their conductivity drops to below 10.sup.-3 s/cm, the threshold below which the process of elimination by diffusion of the anions becomes slower. This is also the case for polyaniline towards 120.degree. C.